The global aim of this research is to explore the potential role of excitatory amino acid systems in acute and chronic actions of opiates. Opiates, such as morphine, are very potent pain relievers and are therefore the drugs of choice for the treatment of severe or chronic pain. These drugs are also widely self-administered, and are therefore important drugs of abuse. Long-term treatment with opiates leads to three well-known consequences: tolerance, which is a decrease in the effect of a drug with chronic use; sensitization, which is an increase in the effect of a drug with chronic use, such that absence of the drug results in an unpleasant withdrawal symptom. These phenomena are important in both the use of opiates for the treatment of pain and in the development of opiate addiction. Recent experiments suggest that an excitatory amino acid receptor, known as the N-methyl-D-aspartate (NMDA) receptor, may have a central role in the consequences of long-term treatment with opiates, including tolerance, sensitization and physical dependence. Additionally, there is increasing evidence that this receptor may also be involved in selected acute actions of opiates, including analgesia and locomoter activity. Further research is necessary, however, to more fully understand the role of NMDA receptors in acute and chronic effects of opiates. The studies outlined in this proposal will examine the ability of NMDA receptor drugs to alter the acute analgesic and the acute locomotor effects of opiates in rats, as well as the consequences of long- term opiate administration, including tolerance, sensitization and physical dependence. If NMDA receptors are indeed involved in these opiate- elicited behaviors, then drugs that block NMDA receptors should alter the acute and chronic effects of morphine and other opiates. The results of these studies will be of theoretical importance, toward a better understanding of the neurotransmitter systems involved in critical behavioral effects of opiates. Moreover, these results may be of clinical relevance by elucidating the potential utility of NMDA receptor antagonists in the treatment of pain and opiate addiction.